 The standard deviation of the facet slopes (gradient of the surface elevation), formula_2 ranges in formula_3.In the Oren–Nayar reflectance model, each facet is assumed to be Lambertian in reflectance.If formula_4 is the irradiance when the facet is illuminated head-on, the radiance of the reflected light formula_5, according to the Oren-Nayar model, is

where

and formula_11 is the albedo of the surface, and formula_2 is the roughness of the surface.